1. Field of the Invention
The present invention relates generally to spools for holding optical fiber, and particularly to spools for holding dispersion compensating fiber.
2. Technical Background
Optical signals transmitted in an optical fiber transmission system typically constitute a series of pulses. Although, within each channel, the pulses are usually centered at a single nominal wavelength, each pulse is actually composed of different spectral components. These spectral components propagate through transmission or typical amplification fibers at different speeds (due to a phenomena referred to as chromatic dispersion). This can result in spectral components of one pulse arriving at a receiver at the same time as the succeeding pulse, thereby degrading the signal to noise ratio. Thus, optical fiber communication systems utilize dispersion compensating modules (“DCM”), to correct for chromatic dispersion. A typical dispersion compensating module includes a housing with one or more spools 10 of dispersion compensating fiber (DCF). This dispersion compensating fiber offsets the chromatic dispersion produced in the transmission and/or amplification fiber. This approach is disclosed, for example, in U.S. Pat. No. 6,456,773 and is illustrated in FIG. 1.
A typical spool 10 of DCF is shown in FIG. 2 and includes a cylindrical hub 12, two flanges 14, 14′, a cover plate 16 and a protective casing 18. The casing 18 covers the fiber and is situated between the flanges. The DCF is wound around the hub 12 between the two flanges 14, 14′ and is protected by the casing 18. The dispersion compensating fiber typically enters into the spool via the underlay groove 15 provided on the internal surface of one of the flanges and exits the spool via the fiber exit or fiber reverse groove 20, provided on the external surface of one of the flanges. The underlay groove 15 is utilized to get the DCF to the hub 12, so as to enable the startup of the winding process. The underlay groove 15 is tangential to the outer diameter of the flange in order to minimize fiber bending. However, this ties the geometry of the underlay groove to the required form factor (i.e., overall size and shape of the package).
After winding is completed, the dispersion compensating fiber exits the spool 10 via the fiber groove 20. In order to protect this fiber a cover plate 16 is then fitted over the external surface (the surface not in contact with the hub 12) of the flange 14′ (and the dispersion compensating fiber contained within the fiber groove 20). In order to minimize fiber bending, and in order to maximize mechanical reliability and life span of DCF, the fiber groove 20 is positioned to allow the DCF to exit substantially tangentially to the radius of the flange. In non-round packages, a mandrel in the box typically reverses the direction of the exiting fiber to align it with fiber entry orientation. In a round package this is done by the reversing fiber groove 20, so that both entering fiber and exiting fiber points in the same direction.
The dispersion fiber containing spools have to be custom made for each customer because each customer requires a different size spool. For example, because the fiber underlay and fiber grooves terminate tangentially to the radius of the flange, the flanges can not be cut down to a smaller size. Thus, features of the plain flange 14, the grooved flange 14′ and the casing 18 are dependent on the required form factor.
The hubs of the spools are typically molded for different thicknesses. The molded parts are then machined to different sizes to maximize the fiber holding capacity for a given form factor. Because the form factor is customer dependent, a standard size hub can not be used. This further increases the cost of the DCM spools.
In addition, if the thermally induced expansion of the hub exceeds a certain value, the DCM turns dark (i.e., stops functioning). Thus, because wound DCF is sensitive to temperature induced stress and has a low coefficient of thermal expansion (CTE), the hubs are usually made from materials with low CTE, for example Macor™, Invar™, or Covar™. During temperature fluctuations, this allows the wound DCF to expand at about the same rate as the hub, minimizing fiber stretching. Macor™ is a glass ceramic with CTE which is similar to that of the DCF. However, it is a very expensive material. Invar™ and Covar™ have a higher CTE than Macor, and are also very expensive.